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Find the mass of one element of a sample

  • Chemistry
  • Thread starter jackthehat
  • Start date
  • #1
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Homework Statement:

A sample of potassium chromate, K2CrO4, is analyzed and found to contain exactly 0.67 grams of potassium. What is the mass of this sample of potassium chromate?

Relevant Equations:

Formula mass = (2 x K) + (1 x Cr) + (4 x O)
1 mole of K2CrO4 has formula mass of ((2 x K) + (1 x Cr) + (4 x O)) g
=> ((2 x 39.0983) + (1 x 51.9961) + (4 x 15.9994)) g = 194.1903 g
we have 2 moles of K in 1 mole of K2CrO4 = (2x 39.0983) g = 78.1966 g
so for 1 mole of substance we have 78.1966 g
So how many moles of the substance do we have if our sample contains just 0.67 g of K
=> 1 mole x (0.67/78.1966) = 0.00856 moles
So the mass of K2CrO4 in 0.00856 moles is ( 0.00856 x194.1903) g
=> Mass of K2CrO4 of this sample is 1.66386 g
Now I was not sure of the number of significant figures required for the solution
since we were originally given the amount 0.67 g of K
I was unsure if this is to 2 or 3 significant figures so I gave the answer to both
I put down 1.66 g (to 3 sig fig) and 1.7 g (to 2 sig fig)
But both answers appear to be incorrect … can anyone help I am stuck ?
 

Answers and Replies

  • #2
etotheipi
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As far as I can tell, your numbers are right. A more direct way to do it might have been to use the ratio $$\frac{2\times 0.39}{2\times0.39 + 52 + 4\times 16} = \frac{0.67}{M}$$
 
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  • #3
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As far as I can tell, your numbers are right. A more direct way to do it might have been to use the ratio $$\frac{2\times 0.39}{2\times0.39 + 52 + 4\times 16} = \frac{0.67}{M}$$
Hi etotheipi,
Thank you for taking the time to look at my problem and for your advice. it has been a help to me to see the problem tackled in an alternative way. much appreciated.
Regards,
jackthehat
 
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  • #4
epenguin
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You meant to write 39 not 0.39?
 

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